Aluminum hydride (“AlH3”) or alane is formed as numerous polymorphs: the alpha (“α”), alpha prime (“α′”), beta (“β”), delta (“δ”), epsilon (“ε”), zeta (“ζ”), or gamma (“γ”) polymorphs. Each of the polymorphs has different physical properties and varying stability. As disclosed in U.S. Pat. No. 6,228,338 to Petrie et al. (“Petrie”) and Brower et al. (“Brower”), “Preparation and Properties of Aluminum Hydride,” J. Am. Chem. Soc., 98(9):2450–2453 (1976), α-alane is the most thermally stable polymorph and its crystals have a cubic or rhombohedral morphology. In contrast, α′-alane forms needlelike crystals and γ-alane forms a bundle of fused needles. γ-alane is produced with the β polymorph, both of which convert to α-alane upon heating. δ-alane and ε-alane are formed when trace amounts of water are present during crystallization. ζ-alane is prepared by crystallization from di-n-propyl ether. The α′, δ, ε, and ζ polymorphs do not convert to α-alane and are less thermally stable than α-alane. Therefore, the α′, δ, ε, and ζ polymorphs are typically not used in explosive or pyrotechnic compositions.
Alane includes about 10% hydrogen by weight and has a higher density of hydrogen than liquid hydrogen. Due to the high hydrogen density and highly exothermic combustion of aluminum and hydrogen, alane is commonly used as a fuel for propellants or as an explosive. When used in a propellant, the alane provides an increased specific impulse compared to propellants that use aluminum alone.
As disclosed in Petrie and Brower, α-alane is typically synthesized by reacting aluminum trichloride (“AlCl3”) and lithium aluminum hydride (“LAH”) in diethyl ether. The aluminum trichloride is dissolved in diethyl ether at −10° C. A minimum of three mole equivalents of LAH is added to the aluminum trichloride solution to produce a solvated alane-ether complex and a precipitate of lithium chloride (“LiCl”). To desolvate the alane-ether complex, 0.5–4 mole equivalents of a borohydride salt, such as lithium borohydride or sodium borohydride, is mixed with the solution including the alane-ether complex. The mixture is filtered and the filtrate is diluted with toluene or benzene to provide an ether to toluene or benzene ratio of 15:85. The mixture is heated to 85° C.–95° C. to desolvate the alane-ether complex and the diethyl ether is subsequently removed by distillation. The precipitated alane is recovered by aqueous acid quenching, filtration, and washing. Brower also discloses that the reaction is conducted in the absence of water, oxygen, and other reactive species because if water is present, the δ and ε polymorphs are undesirably formed.
However, the borohydride salts used to desolvate the alane-ether complex are expensive and are not recovered, making this synthesis of α-alane expensive. The borohydride salts also generate byproducts that require disposal. Furthermore, the alane produced by the method of Petrie or Brower is typically contaminated with undesirable polymorphs and is prone to decomposition during heating. More importantly, since the α-alane is contaminated with the other alane polymorphs, this method of producing α-alane gives variable and irreproducible results.
Alane may also be synthesized from aluminum and hydrogen at a high pressure (0.5–6.5 GPa) and temperature (100° C.–700° C.), as disclosed in Konovalov et al., “High Pressures in the Chemistry of Beryllium and Aluminum Hydrides,” Russian J. Inorg. Chem., 37(12):1361–1365 (1992). However, preparative quantities of the alane are not produced by this synthesis due to the difficulty of creating gas holders for the hydrogen.
It would be desirable to reproducibly produce a high yield of α-alane using a low-cost synthetic method.